There are a number of industries which have evolved to integrate complex industrial processes with distributed computerized control systems for those processes. Two such examples are the oil refining industry and the power generating industry. Others include a wide variety of manufacturing facilities, including for example paper manufacturing facilities. These industries utilize process control systems including control elements that are distributed, but the operation and control of the separate elements is, by necessity, highly integrated.
The integrated industrial control systems have evolved to include distributed control processors at localized industrial equipment sites. The distributed control processors communicate with industrial process control hardware in the system and with one or more control system operator consoles (workstations) at a central operating station. The control processors communicate with industrial process equipment such as pumps and furnaces to set/sense particular values affecting/representing the state of a controlled process. Examples of such processor control systems are the I/A SERIES (Registered Trademark of The Foxboro Company) industrial process control systems. The I/A SERIES industrial process control systems incorporate a Nodebus LAN link (IEEE 802.3) to communicatively connect workstations, gateways, and control processors. The control processors, in turn, are connected to fieldbus modules via a fault tolerant fieldbus link (IEEE 1118).
The distributed control system components of the I/A SERIES industrial control systems communicate at a local level with a set of special purpose process control nodes to receive a set of process values representing the state of a controlled process. At a higher level, the control processors communicate with other control processors and workstations over a Nodebus LAN. Workstations and control processors execute software at the distributed locations to render a set of control values based upon the set of process values.
Process control systems are generally arranged into regulatory and supervisory levels. Regulatory control blocks reside at the lowest level of the process control hierarchy. The regulatory control blocks are responsible for input and output signals received from and transmitted to field devices including sensors and control elements. PID control blocks, also considered regulatory control blocks, respond to sensed process variables according to supplied setpoint values and render a correction output value. Regulatory control blocks generally reside in control processors, or even in field devices.
Supervisory programs reside at a relatively higher level of the process control system. Supervisory control programs coordinate the operation of the multiple control points in a process control system. Supervisory control logic establishes setpoint values for the PID control blocks and other regulatory control blocks, and implement an overall process control strategy that may itself be carried out at several levels of operation. In order to ensure that the finite computation capacity of control processors is not exhausted by supervisory control program execution, supervisory control is generally executed within workstations that are connected to control processors via network links. The output values of supervisory control programs are transmitted to the appropriate control processors for use in the PID and other regulatory control blocks.
Control processors today, having both greater primary memory capacity and increased central processor speed, are considerably more powerful computers than their predecessors of the early 1990's. The increased computation power of today's control processors has created interest in migrating supervisory tasks from workstations to the control processors for which the supervisory tasks are executed. Under such a distributed processing scheme, supervisory and regulatory control tasks for a controlled process are executed at the local level by the distributed control processors connected to a control network. The workstations in the control network provide a user interface and supporting programs enabling a user to view the status, and modify the execution, of the controlled process.
Several advantages result from such a control distribution scheme. Network traffic decreases for a given control strategy, since a workstation is not called upon to regularly compute and update setpoint values stored at a control processor node in a process control network. As a result, the maximum size of a process, measured by the number of control points, under the control of a single network may increase due to the reduced control processor/workstation communications. Since control processor-to-workstation communications are greatly reduced, a supervisory control program in the control processor may potentially respond faster to changes in dynamic parameters of the controlled process, thereby facilitating tighter control over process elements. Control over the process is fault-tolerant, since a control processor will continue to control the process without interruption in the event that the communication path between the control processor node and its corresponding workstation is rendered inoperative.
The role of the workstations in providing real-time control commands would be reduced if supervisory control tasks are shifted to the control processors. Operation complexity of the workstations may decrease because the importance and frequency of use of the communication path to the control processor(s) is reduced. The workstations would be freed from real-time short-cycle-time tasks to provide new and enhanced user interface facilities and process configuration tools.